199 The effect of different doses of follicle-stimulating hormone during superovulation on ovarian function in dairy cattle

Cattle with an unknown antral follicle count (AFC) during follicular waves respond to different FSH doses during superovulation in a curvilinear fashion with the highest doses decreasing or not increasing ovulatory follicle number, number of transferable embryos, and number of corpora lutea (CL). These observations support the hypothesis that heifers treated with different FSH doses during superovulation will reach a superovulation maximum (SOVmax) and that doses exceeding SOVmax are excessive, resulting in a decrease in ovarian function. To test this hypothesis, we used eight 12-month-old Holstein heifers with a low AFC (<10 follicles ≥3mm in diameter, index for small ovarian reserve). These 8 heifers were subjected to 4 different superovulations with a 21-day intervals between each superovulation. During each superovulation, which began on Day 1 of a synchronized oestrous cycle before selection of a dominant follicle, 2 of the 8 heifers were treated (twice-daily FSH injections×4 days) with 20, 40, 80, or 120mg of Folltropin-V (Vetoquinol, Lure, France). At the end of the study, the same 8 heifers had been superovulated 4 times with each of the 4 FSH doses. To enhance ovulation rate in response to hCG, each superovulated heifer was injected with 3 different injections of prostaglandin F2α (PGF2α) spaced 12h apart to induce luteolysis of the original CL. Two PGF2α injections were given after each FSH injection on the last day of FSH treatment and the last PGF2α injection was given after the hCG (2500IU) injection, which was 12h after the last FSH injection. Daily ultrasonography was used to measure AFC and number of ovulatory follicles and CL during the study. Results of t-test analysis showed that at time of hCG, AFC was greater (P>0.05) for heifers treated with the 40-mg (mean±s.e.m., 27±3.7) v. 20-mg (17.9±2.5) FSH dose, but the response to higher FSH doses (27.9±4.4; 27±3.3) did not differ from the response to the 40-mg dose. Number of ovulatory follicles was greater (P<0.05) for heifers treated with the 40-mg (17.5±2.3) v. 20-mg (10.1±1.4) FSH dose, but the response to higher doses (18.9±3.3; 17.3±2.4) did not differ from the response to the 40-mg dose. At Day 7 post-hCG, CL number was greater (P<0.05) for heifers treated with the 40-mg (13.8±2.9) v. 20-mg (7.4±1.4) dose, but response to higher doses (9.9±3.2; 9.1±2.3) did not differ from the response to the 40-mg dose. Ovulation rate (ovulatory follicle number divided by CL number) was greater (P<0.05) for heifers treated with the 40-mg (79±0.08%) v. 80-mg (52±0.09%) or 120-mg (53±0.09%) dose, but similar to that of heifers treated with 20-mg (73±0.09%) FSH dose. These results support the conclusion that the 40-mg FSH dose achieves SOVmax and FSH doses greater than SOVmax in cattle with a low AFC and small ovarian reserve are excessive and detrimental to ovarian function. This project was supported by Agriculture and Food Research Initiative Competitive Grant no. 2017-67015-26084 from the USDA National Institute of Food and Agriculture (Washington, DC).